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Chevrons are not ribs [was Re: Spare ribs?]

Matt Bonnan had written:
<< Chevrons are not homologous to ribs, however, because they are essentially modified haemal arches and arise below (ventral to) the bodies (centra) of the vertebrae. Ribs articulate with the centra and transverse processes instead. >>

Dinogeorge wrote:

"If you look at the skeletons (such as they are) of the most primitive living

vertebrates, it seems to me that the cervical ribs, thoracic/abdominal ribs,
and hemal arches form a pretty continuous series along the vertebral column,
and that the details of how and where they articulate with the vertebrae in
various places along the column are secondary. This leads me to suspect that
they arise in the same way from the same precursor structures during the
ontogenetic development of the vertebrate body. <snip> ... but it seems unparsimonious for
vertebrates to develop ribs in one set of loci and hemal arches independently
in another set of loci where the ribs are not found. The only other thing I
can think dinosaurian hemal arches might be is modified intercentra, which

seems possible but weird."

According to Romer, Osteology of the Reptiles, 1956, pgs. 221-222 (my itialics and caps):

"The intercentrum, the major ventral element of the vertebra in reptile ancestors, had already undergone considerable reduction inmany of the older reptiles of the Late Paleozoic; and in later forms it disappears althogether over most of the column.  It is properly a part of the vertebral segment to which the centrum behind it also belongs. <snip>

"Primatively, the major rib head (capitulum) articulated with the intercentrum, and a small articular area, distinct from the general surface of the bone, is sometimes seen in early reptiles.  In general, however, this articulation tended to shift toward or onto the centrum ...

"In the caudal vertebrae (except for the first few immediately behind the sacrum), there is usually present in each segment a ventral extension from the intercentrum, the chevron or haemal arch.  This is somewhat comparable TO THE NEURAL ARCH ABOVE and, like the caudal neural arch, forms part of a line of division between the caudal muscles of the two sides of the tail."

Thus, the ribs and haemal arches/chevrons are not homologous to one another.  In fact, we usually reserve the term homologous for the same structure in different organisms that suggests descent with modification from a common ancestor.  The humerus of a human, whale, and bat are all homologous, but ribs and chevrons in the same animal (in this case, a dinosaur), even if they arose from similiar embryological origins, would not be homologous in the strictest sense of the word.  We would, instead, call similar, repeating segments of bone or muscle serially homologous, and it is in this sense I think George is using the word homologous. 

The haemal arches/chevrons form as a ventral extension from the intercentrum, but they are not modified intercentra, per se.  If one were going to stab at similarity, the chevrons are kind of like upside down neural arches, as Romer suggests, but again, there is probably not a common developmental locus at work here.  Although chevrons are serially homologous structures, like ribs, this does not necessarily mean the most parsimonious explanation for their formation is that they develop from the same locus or loci as ribs in the reptilian embryo.

Because chevrons do not form a continuous series from underneath the sacrum to the tail, and because several proximal caudal vertebrae do not have chevrons, this would be evidence against a single developmental locus for both ribs and chevrons.  And, because chevrons do not articulate with the same portions of the vertebrae as the ribs do, this is evidence against common embryological development, although it does not reject this hypothesis outright.  In any case, the details of articulation of elements are not secondary considerations when arguing for skeletal homology, serial homology, or common embryological origin.  Rather, they are crucial clues (perhaps the only clues we have in paleontology!) to deciphering skeletal relationships and development.

Matt Bonnan

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